138 Mr. Troughton on dividing Instrument's. 
Aberdeen. I notice the two latter, principally to give myself 
an opportunity of saying that, if those three scales were to be 
compared together, notwithstanding they were divided at dis- 
tant periods of time, and at different seasons of the year, they 
would be found to agree with each other, as nearly as the 
different parts of the same scale agree. 
I hope I may here be allowed to allude to an inadvertency 
which has been committed in the paper mentioned above ; and 
which Sir George intended to have corrected, had he lived 
to conclude his useful endeavours to harmonize the discordant 
weights and measures of this country. The instruments which 
he has brought into comparison are, his own five feet standard 
measure and equatorial ; General Roy’s forty-two inch scale ; 
the standard of Mr. Aubert ; and that of the Royal Society. 
The inadvertency is this : In his equatorial, and the standard 
of the Royal Society, he has charged the error of the most 
erroneous extent, when compared with the mean extent, alike 
to both divisions ; i. e. he has supposed one of the divisions, 
which bound the erroneous extent, to be too much to the 
right, and the other too much to the left, and that by equal 
quantities : This is certainly a good natured way of stating 
the errors of work ; and perhaps not unjustly so, where the 
worst part has been selected ; but, in the other three instances, 
namely, in General Roy’s, Mr. Aubert’s, and his own stan- 
dard, he has charged the whole error of the most erroneous 
extent to one of the bounding lines. 
I was well confirmed in my high opinion of the general 
accuracy of Bird’s dividing, when, last winter,* I measured the 
chords of many arcs of the Greenwich quadrant : That in- 
strument has indeed suffered both from a change in its figure, 
* This paper was written in June 1808. 
